Conventional lithographic imaging processes employ liquid immersion to increase the effective numerical aperture (NA) and make use of extensive resolution enhancement techniques (RET). This appears adequate to meet the lithographic needs of integrated circuits through the 32 nm generation. Starting from the 22 nm node, the numerical aperture NA has only marginally increased and the k1 value is approaching the theoretical limit of 0.25. Further lithography scaling has been relied mainly on double or even multiple patterning techniques. While multiple patterning techniques provide the resolution required for further scaling, the overall cost to implement multiple patterning techniques has reached a level that other techniques may need to be explored.
Directed self-assembly (DSA) has recently emerged as such a technique for lithographic patterning to reach 22 nm and below. In self-assembly, the formation of features of fine geometric dimensions occurs not through external patterning, but through the spontaneous phase behavior found among polymers on the molecular level. Of particular interest are diblock copolymers formed by chemically connecting normally incompatible species, such as poly(styrene) (PS) and poly(methyl-methacrylate) (PMMA). By creating linked chains of these materials and controlling the relative molecular weight, various structures can spontaneously form. The appeal of these structures, easily formed around 20 nm in size, is that the boundaries between the two disparate polymers can be quite uniform, with the uniformity dictated not by the noise properties of a patterning process, but by the relative molecular weight of the polymers. This is a quantity that can be precisely controlled.
One problem with these polymers is that, although local order and roughness can be quite good, small variations in the polymer chain can lead to kinks that reset the self-assembly process. Long range patterns therefore appear somewhat chaotic. The problem of long range order can be addressed by using a conventional patterning process to guide and direct the spontaneous formation of the block co-polymer structures. This “directed self-assembly” can take the form of having the self-assembly occur in grooves (grapho-epitaxy) or other geometrically confined regions or by chemically patterning a surface to create local affinities to the various portions of the block co-polymers (chemo-epitaxy).
In a grapho-epitaxy process, the main mechanism by which the block-copolymer self organizes in useful domains, is dominated by the concept of confinement. Neutral walls or pillars prevent certain chain configurations which then lead to the polymer to adjust its periodic structures along a pre-determined axis. The benefit of this technique is that the guiding pattern can be very local and there is limited interdependency between different organization domains.
A chemo-epitaxy process defines the preferred direction by a chemical brush which changes the surface energy of the substrate, by doing so and due to the different chemical affinity of the different diblock species to the substrate, the material organizes in a preferential direction minimizing the energy required to achieve a specific configuration. The benefit of this technique is the ability to pattern dense gratings or arrays as the guiding patterns are underneath the block-copolymer.
A lot of progress of DSA in getting impressive resolution has been repeatedly demonstrated using the PS-PMMA system. Some record low pitches of sub-15 nm have been observed in laboratory recently. DSA also has an advantage of not requiring new capital equipment investment and can be used complimentary with other lithographic techniques, including EUV and Direct Write Electron Beam (Ebeam). Using a grapho-epitaxy process for contact and via layers seems like a promising technique as it has the potential to reduce total mask count and remove one patterning step from the process of record while maintaining the yield. However, DSA has its own set of challenges that need to be addressed before becoming a viable alternative to current lithographic techniques.